1. Field of the Invention
The present invention relates to a power supply circuit which produces a dc output through a process of DC-DC conversion, and more particularly to such a power supply circuit of a ringing choke converter type adapted in use to charge a battery.
2. Description of the Prior Art
It is generally recognized that a DC-DC conversion system is suitable for obtaining a desired dc power output from a dc voltage power source, and useful in providing a power supply circuit for charging a battery. In this accord, there has been a DC-DC converter generally referred to as a ringing choke converter (RCC) as disclosed in U.S. Pat. No. 4,376,263. This type of DC-DC converter produces an output as a form of a released energy from an output winding of a transformer by blocking oscillation. In the circuit, a feedback current is fed to the base of a switching transistor for controlling the ringing choke converter in a positive feedback manner. However, since the feedback current is made from the entire output winding of the transformer, the total induction voltage of the output winding is to be applied across a resistor and capacitor combination inserted in series between the output winding and the base of the switching transistor. This means the resistor and the capacitor are to have great heat and voltage endurances respectively.
To this end, there has been proposed a scheme to provide a feedback winding other than the output winding. The scheme is exemplarily shown in the circuit diagram of FIG. 8 for easy understanding thereof. The circuit operating from a dc power source S to charge a battery B comprises a switching transistor 101, and a transformer 110 having an input winding 111, an output winding 112, and a feedback winding 113. The circuit further comprises a starting resistor 121, a current regulating resistor 123, and a speedup capacitor 132.
The provision of the feedback winding 113 produces the effect that the feedback current fed to the base of the switching transistor 101 is reduced to such an extent as to allow the use of smaller electric components as the resistor 123 and the capacitor 132 acceptable for a compact circuit designing purpose. With this circuit, it may be possible to produce a larger charge current by increasing the feedback current, i.e., the base current of the switching transistor. However, this eventually leads to the increase in bulk and weight of the electric components and therefore has a certain limitation as to the output obtained from the standpoint of designing a compact and cost effective circuit arrangement. In this respect, the circuit is not satisfactory to produce a greater output at a minimum bulk and weight requirement to the electric components.